In recent years, many electrically powered vehicles are used, including electric vehicles whose drive source is a motor and hybrid electric vehicles whose drive sources are an engine and a motor. Such an electrically powered vehicle incorporates a chargeable and dischargeable secondary battery such as a lithium-ion battery. It is known that such secondary batteries degrade as they are repeatedly charged and discharged, and the degradation gradually lowers the battery capacity or increases the internal resistance. Such degradation of secondary batteries includes high-rate degradation and material degradation. The high-rate degradation proceeds as a secondary battery is repeatedly used with a large charging or discharging current, or at a high rate. The high-rate degradation is a phenomenon in which the concentration of salt in a liquid electrolyte in a secondary battery becomes uneven when a large discharging current or a large charging current flows, and the gradient in the concentration of salt causes an increase in the internal resistance. The material degradation is degradation of material in constituent components of a secondary battery, and occurs depending on the electric current value or the temperature.
To make better use of a secondary battery, techniques for monitoring the amount of damage resulting from the high-rate degradation or the amount of damage resulting from the material degradation have been proposed. For example, JP 2013-225397 A discloses a technique of monitoring the amount of high-rate degradation damage and the amount of material degradation damage to thereby lower the upper limit of discharge power when the amount of high-rate degradation damage exceeds a predetermined threshold, in which the threshold is changed in accordance with the amount of material degradation damage. By employing this technique, the progress of the high-rate degradation is retarded because discharge power is restricted if the amount of high-rate degradation damage becomes too high.
However, according to JP 2013-225397 A, although discharge power is restricted when the amount of high-rate degradation damage exceeds the threshold, control for retarding the high-rate degradation is not specifically performed until the amount of high-rate degradation damage exceeds the threshold. The technique of JP 2013-225397 A therefore has a problem in that fuel efficiency or vehicle performance is reduced as the restriction of discharge power is started relatively easily.